This invention relates to apparatus for molding thermoplastic pipes, externally ribbed or corrugated.
Known such apparatus (U.S. Pat. Nos. 3,998,579, 4,021,178 and 4,504,206) include a pipe die for extruding a parison of a thermoplastic material, and moldblock halves circulating in an endless path containing a straight forward run wherein the halves cooperate with each other in the directions of their circulation and perpendicularly to the latter, to form a travelling mold for the thermoplastic pipe, a return run and two semi-circumferential guide tracks for transferring the halves between said runs.
The apparatus of the first known patent is particularly adapted to produce externally ribbed pipes. That is why it has a means for withstanding the hydraulic pressure of molding acting to separate the halves in the straight forward run in said perpendicular direction.
In that apparatus said means constitutes two guiding rails biased toward each other by means of springs. The drawback of such a design lies in extremely high loads and wear in the rails since the hydraulic pressure of the injection molding acts in the frame-springs-rail-halves-rail-frame zone. Also, the patent does not states whether the halves are independent (chainless) or are parts of chains. The chainless apparatus with independent halves (described in the next two patents) are designed for molding of corrugated pipes and do not have said means and, in addition, cannot withstand the hydraulic pressure of molding acting to separate the halves in the direction of their circulation. That is why, such apparatus cannot be used successfully in the molding of externally ribbed pipes.
Similar problems can arise in blow molding of extremely large corrugated pipes.